FIG. 8 shows a background-art general example of so-called axial magnetic bearing apparatus for bearing a rotating shaft of rotating apparatus such as an electric generator, an electric motor, or the like, in a thrust direction by use of magnetism. In the drawing, the reference numeral 1 represents a rotating shaft, to which a rotary disc 2 made of a magnetic material is fixedly attached. The rotary disc 2 usually has sleeves 6 formed on opposite sides of the rotary disc 2 so as to make the fixation of the rotary disc 2 to the rotating shaft 1 firm. The reference numeral 10 represents each of ring-like electromagnetic coils formed by winding coated copper wire around the rotating shaft 1 with a required and adequate number of turns. Each of the electromagnetic coils 10 is incorporated in an electromagnetic stator 7 having an inside magnetic pole tooth 11 and an outside magnetic pole tooth 12. The reference numeral 8 represents each of ring-like housings which forms a magnetic circuit portion for corresponding one of the electromagnetic stators 7. The electromagnetic coils 10 are received in coil slots 9 formed symmetrically with respect to the rotation axis of the ring-like housings 8. The electromagnetic stators 7 are paired and disposed in opposition to each other with respect to a collar 22 so as to have suitable very small distances from the rotary disc 2 respectively on opposite sides of the rotary disc 2. Thus, the electromagnetic stators 7 are attached to the casings 23.
This axial magnetic bearing apparatus is controlled as follows. That is, axial displacement of the rotating shaft 1 is measured by a not-shown displacement sensor. On the basis of an output signal from this displacement sensor, electric currents to the electromagnetic coils 10 are adjusted to suitably vary magnetic attraction force acting between the rotating disc 2 and the inside magnetic pole teeth 11 of the electromagnetic stators 7 and between the rotating disc 2 and the outside magnetic pole teeth 12 of the electromagnetic stators 7. Thus, the rotating shaft 1 is borne at a target position distant from the electromagnetic stators 7 and in non-contact therewith.
However, in the structure of the above-mentioned axial magnetic bearing apparatus generally used in the background art, for example, magnetic circuits as shown in FIG. 9 are formed among the two electromagnetic stators 7a and 7b and the rotary disc 2 by selecting the polarities of the electric currents flowing into the electromagnetic coils 10. At this time, there are two magnetic circuits 13 formed between the respective electromagnetic stators 7 and the rotary disc 2, and a magnetic circuit 14 formed between the two electromagnetic stators 7a and 7b opposed to each other with respect to the rotary disc 2. Here, the magnetic circuits 13 are magnetic circuits which contribute to magnetic attraction force required for the position control of the axial magnetic bearing, but the magnetic circuit 14 is a magnetic circuit which does not contribute to the magnetic attraction force at all.
As a result, the magnetic attraction force generated by each of the electromagnetic stators 7 decreases so that the support stiffness of the axial magnetic bearing apparatus decreases.
Incidentally, the reason why the support stiffness decreases due to the generation of the magnetic circuit 14 is, for example, disclosed in Japanese Patent Laid-Open No. 122896/1993. Therefore, the description of the reason is omitted here.
Thus, an invention for improving this defect is, for example, disclosed in Japanese Patent Laid-Open No. 122896/1993. FIG. 11 shows this background-art improved axial magnetic bearing apparatus. In the drawing, one rotary disc piece 3 made of a magnetic material has an L-shaped sectional structure with a sleeve 6. A pair of such rotary disc pieces 3 are opposed to each other on their contra-sleeve sides, and a disc 5 of non-magnetic material is sandwiched like a layer between the rotary disc pieces 3. Thus, one rotary disc 2 is formed. Electromagnetic stators 7 are disposed respectively with suitable very small distances from the rotary disc 2 on opposite sides of this rotary disc 2 so as to be opposed to each other with respect to a collar 22. Thus, the electromagnetic stators 7 are attached to casings 23.
Accordingly, a magnetic circuit 14 which is formed through the two electromagnetic stators 7 opposed to each other with respect to the rotary disc 2 and which does not contribute to magnetic attraction force is eliminated. On the other hand, independent magnetic circuits 13 are formed between the respective electromagnetic stators 7 and the rotary disc 2. Thus, the performance of position control of the axial magnetic bearing apparatus is improved.
Now, generally, axial magnetic bearing apparatus is often used as a support mechanism for a high-speed rotating body. It is difficult to realize such a support mechanism by a mechanical contact type bearing. In the high-speed rotating body, the natural frequency of the first-order bending mode of a rotor is important when the dimensions and shape of the rotor are designed. It is requested to design the rotor to have a natural frequency as high as possible. To this end, the strength and mass of the rotary disc 2 and the fixation stiffness between the rotating shaft 1 and the rotary disc 2 often become critical in the axial magnetic bearing apparatus which generally has a maximum outer diameter in the rotor shape. It is therefore necessary to pay close attention to the design of the rotor shape, particularly the design of the shape of the rotary disc 2 of the axial magnetic bearing apparatus.
Generally, when a rotor makes a rotary motion, centrifugal force F [N] as shown in the following expression acts on the rotating body, and the magnitude thereof is in proportion to the mass and the outer diameter of the rotating body.F=mrω2 
Provided that m designates the mass [Kg] of the rotating body, r designates the outer radius [m] of the rotating body, and ω designates the rotation angular velocity [rad/sec].
The rotary disc 2 of the above-mentioned improved axial magnetic bearing apparatus (FIG. 11) in the background art has two rotary disc pieces 3. Each of the rotary disc pieces 3 has an L-shaped sectional structure with a sleeve 6. The two rotary disc pieces 3 are opposed to each other on their contra-sleeve sides, and a non-magnetic disc 5 is sandwiched between the rotary disc pieces 3 so as to form one rotary disc. Thus, the two rotary disc pieces 3 and the non-magnetic disc 5 are independent of one another, and not locked to one another. As shown in FIG. 12, at the time of high speed rotation, centrifugal force 24 acts on the rotary disc 2 and the non-magnetic disc 5. Thus, the rotary disc 2 has a maximum outer diameter at rotation-axis-direction positions of angular portions 4 of the rotary disc pieces 3. As a result, larger centrifugal force acts on the rotary disc 2 at the positions than in any portion of the sleeves 6. Thus, maximum stress is generated in the angular portions 4 due to the centrifugal force at the time of high speed rotation. A gap 25 between the rotating shaft 1 and the rotary disc 2 becomes maximal at the positions of the angular portions 4. Thus, there arises a problem that the fixation between the rotating shaft 1 and the rotary disc 2 is retained only at a part of the sleeves 6. On the other hand, the same thing can be applied to the non-magnetic disc 5 . Since the non-magnetic disc 5 has a maximum outer diameter, there is produced a gap between the rotating shaft 1 and the inner diameter of the non-magnetic disc 5 so that the fixation cannot be retained perfectly.
Further, the natural frequency of the first-order bending mode of a rotating body is generally in proportion to the square root of the reciprocal of the rotor mass. It is therefore advantageous that when a high-speed rotation rotor is designed, the number of fixation parts resulting in additional mass to thereby cause the decrease in the rotor stiffness is reduced to the utmost so that the weight of the rotor is reduced. However, the rotary disc 2 of the above-mentioned background-art improved axial magnetic bearing apparatus has a structure in which the rotary disc 2 is divided into two pieces, and the non-magnetic disc 5 is added between the two rotary disc pieces 3. Thus, there has been also a problem that such a structure is disadvantageous because the number of parts is increased, and the rotor mass is also increased.
In addition, another embodiment of the above-mentioned background-art improved axial magnetic bearing apparatus has a structure in which an air layer in place of the non-magnetic disc 5 is sandwiched between the two rotary disc pieces 3. Also in this case, similarly to the above-mentioned embodiment, the two rotary disc pieces 3 are independent of each other and not locked to each other. Thus, the fixation of the angular portions 4 is spoiled due to centrifugal force at the time of high speed rotation. As a result, a maximum gap is generated so that the fixation between the rotating shaft 1 and the rotary disc 2 is retained only at a part of the sleeves 6.
That is, there has been a problem that it is difficult to apply the above-mentioned background-art improved axial magnetic bearing apparatus to a mechanism for supporting a high-speed rotating body.
In addition, typically, an iron-based magnetic material is often used for the casings 23 and the collar 22 to which the electromagnetic stators 7 are attached, from the point of view of the manufacturing cost, the cutting workability, and so on. Thus, in the structure of the background-art axial magnetic bearing apparatus (FIG. 8), a magnetic circuit 15 as shown in FIG. 10 may be formed. Incidentally, though not shown, also in the improved thrust bearing apparatus shown in FIG. 11, a similar magnetic circuit 15 is formed when the casings 23 and the collar 22 are formed out of an iron-based magnetic material. At this time, there are two magnetic circuits 13 formed between the respective electromagnetic stators 7 and the rotary disc 2, and a magnetic circuit 15 formed among the two electromagnetic stators 7 opposed to each other with respect to the rotary disc 2, and the collar 22 or the casings 23. Here, the magnetic circuits 13 are magnetic circuits which contribute to magnetic attraction force required for the position control of the axial magnetic bearing, while the magnetic circuit 15 is a magnetic circuit which does not contribute to the magnetic attraction force at all.
Accordingly, also in this case, the magnetic attraction force generated by each of the electromagnetic stators 7 decreases so that the support stiffness of the axial magnetic bearing decreases.
The present invention was devised to solve the foregoing problems. An object of the invention is to provide axial magnetic bearing apparatus having a structure in which formation of any magnetic circuit not contributing to position control of the axial magnetic bearing is relieved or prevented to provide high efficiency and superior controllability, and having a structure in which the stiffness of a rotor is not lowered even at the time of high speed rotation.